Displacement pumps which are designed for slurry pumping are encountered in mainly two designs, either as piston pumps preferably double-piston pumps, or as plunger pumps. Piston pumps are considered to be most suitable for the pressure range up to 200 bar and for less abrasive media, whilst the main sphere of application for plunger pumps is the pressure range 250-300 bar and for abrasive media. The reason for plunger pumps being more suitable for high pressure is connected with their generally more robust construction (solid plunger piston), whilst better resistance to wear is brought about by the possibility of simply introducing water purging of the single-acting plunger. However it is known, from German Offenlegungsschrift No 2 552 828, that it is possible to introduce water purging also for piston pumps, although the technique illustrated in this patent publication has not been widely adopted. This can be regarded as being due to the fact that the design otherwise has a number of imperfections and disadvantages. Thus major technical problems are encountered with piston sealing, piston rod sealing and the cylinder bore in piston pumps, which among other things is connected with the fact that the pump is driven by an external motor via a piston rod which extends through the pump housing. With plunger pumps, as in accordance with U.S. Pat. No. 2,836,122, the plunger and plunger seal represent critical wear components.
A hydraulic drive pump of the displacement type is already known, e.g. from Swedish Pat. No. 412 939. With this pump it is possible to eliminate or restrict the above mentioned disadvantages of piston and plunger pumps. Thus this pump signifies a major technical advance. However in its technical design it differs radically from pumps of the piston or plunger type in that it operates with hose pump elements.
Further a hydraulic piston pump for the pumping of viscous, pulpy or plastic substances and particularly concrete is known from U.S. Pat. No. 3,146,721. In this pump solid particles may pass the pump piston sealing to the spece between the pump piston and the rear gable of the pump cylinder, which space is filled with flushing water under atmospheric pressure. The intention is that these particles shall be rinsed away from the flushing water section in connection with the return stroke (suction stroke) of the pump piston. The hydraulic piston is arranged in a separate hydraulic cylinder which is partitioned from the flushing water section of the pump by the said gable. The hydraulic piston and the pump piston are connected with one another by a piston stem extending through a seal in the gable, and the pump chamber is partitioned from the hydraulic section by the intermediate flushing water section which always is at zero pressure. Thus the pump piston sealing is not at a balanced pressure, that is to say the pressure difference over the seal corresponds to the full work pressure of the pump. Moreover the flushing system is designed only to rinse away such particles which have passed the pump piston sealing, which means that the pump piston sealing in no particular way is protected against wear and possible damage caused by particles in the pumping substance which are in direct contact with the sealing.